1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image on a recording medium between a heating member and a pressuring member, and to an image forming apparatus, such as a copier or a laser printer, disposed with this fixing device.
2. Description of the Related Art
In image forming apparatus such as copiers and printers, an image carrier such as a photoconductor is charged and irradiated with laser light such that an electrostatic latent image is formed on the surface of the image carrier. The electrostatic latent image is made visible by a developing device, and a toner image comprising powder toner is formed. Then, the toner image is transferred to a recording medium such as paper, and thereafter the toner image on the recording medium is fixed onto the recording medium by a fixing device, whereby an image is formed.
The fixing device includes a heating roller, which comprises a rotor disposed with an internal heater, and a pressuring roller, which presses the recording medium against the heating roller. The recording medium on which the toner image has been formed is passed between the heating roller and the pressuring roller, and the toner image is fused and fixed onto the recording medium as a result of being heated and pressured. Sometimes, in consideration of maintenance and the like, the fixing device is configured as a unit that is attachable to and detachable from the image forming apparatus.
In such a fixing device, a recording medium such as an envelope sometimes becomes wrinkled when a heavy load is applied thereto when the toner image on the recording medium is fixed at the portion where the heating roller and the pressuring roller press the recording medium. For this reason, fixing devices have conventionally been proposed where the load of the pressuring roller can be varied manually. However, when the load of the pressuring roller is manually varied by the user, the recording medium cannot be completely prevented from becoming wrinkled, and the recording medium can end up becoming wrinkled due to mistakes or the like.
In Japanese Patent Application Publications (JP-A) Nos. 8-69202 and 2001-249569, fixing devices have been proposed which are configured to automatically adjust the load of the pressuring roller pressing against the heating roller by causing a swinging arm that presses a support shaft of the pressuring roller to swing. However, these fixing devices have many parts and expensive configurations because they include various kinds of parts such as a one-way clutch, a motor and a cam floor.
In Japanese Patent Application Publication No. 2004-109521, a fixing device has also proposed where the number of parts is reduced by disposing a reference position changing cam. With this configuration, the cost can be reduced, but the ease with which the unit can be loaded into and unloaded from the image forming apparatus and the stability in the precision of the unit and cam position are not taken into consideration.
That is, among conventional fixing devices, there have been none that can satisfy the stability in the precision of the unit and cam position while maintaining the ease with which the unit can be loaded into and unloaded from the image forming apparatus body.